Note: Descriptions are shown in the official language in which they were submitted.
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IRRADIATING ~<ESIN-COATED OPTICAL FIBER THROUGH LIGHT-
ABSORBING FLUID MATERIAL
The present invention relates to a method for pro-
ducing an optical fiber. More particularly, it relates
to a method for producing an optical fiber having a
surface coating of an ultraviolet (UV)-curable resin.
Conventionally, optical fibers naving coating layers
made of a W -curable resin are produced by means of an
apparatus which includes a W irradiating furnaceO
The conventional apparatus is described in more detail
with reference to the accompanying drawings, in which:
Fig. 1 schematically shows a conventional apparatus
for producing an optical fiber having a coating layer of
a W -curable resin;
Figs. 2A and 2B are a perspective view and a cross
sec~ional view, respectively~ of the conventional W
15 irradiating furnace of Fig. l;
Fig. 3 schematically shows an apparatus for producing
an optical fiber having a coating layer of the UV-curable
resin according to the present invention;
Figs. 4A and 48 are a perspective view and a cross
20 sectional view, respectively of a W irradiating furr.ace
used in the apparatus of Fig. 3 according to the present
invention;
Fig. 5 shows a system for controlling flow rates of
gasses supplied to the W irradiating furnace;
,
:, ..
.
~3~f~
Fig. 6 shows the change of the transmission of W
light as time passes according to the present invention;
FigO 7 shows the change of the transmission of W
light as time passes according to the conventional method;
Fig. 8 shows the relationship between the transmission
of W light and the molar fraction of chlorine in a mixture
of nitrogen and chlorine; and
Fig. 9 shows the change of the molar fraction of
chlorine in a mixture of nitrogen and chlorine as time
passes.
In the apparatus of Fig. 1, a preform 1 for an opti-
cal fiber is melted by a heater 2 ~for example, a carbon
resistance heater) and drawn and spun to form an optical
fiber 3. The optical fiber 3 is passed through a resin
coating cup 4 in which a W -curable resin is applied to
the surface of the optical fiber 3 and then passed through
a W irradiating furnace 5 to cure the resin. The optical
fiber coated with the cured resin is wound up by a reel 11
through a capstan 10.
As shown in Figs. 2A and 2B, the W irradiating
furnace 5 comprises an ellipsoidal reflection cylinder
6, a W lamp 7 and a quartz tube 8 for passing a gas
which permits full curing of the W -curable resin to
take place. The W lamp 7 and the quartz tube 8 are so
arranged that their axes are aligned with each o~
the Eocus lines 6a and 6b of the ellipsoidal cylinder 6.
~hen the optical fiber 3 coated with the W-curable resin
is passed through the quartz tube 8, the UV-curable resin
is cured by the focused W radiation from the W lamp 7.
In the conventional method as described above, a
proportion of the W -curable resin is evaporated by heat
generated by the W irradiating furnace when the optical
fiber 3 ls passed through the quartz tube 8 and the eva-
porated resin becomes adhered to and baked on the inner
surface of the quartz tube and eventually becomes car-
bonized. Consequently, the amount of W light which
~3~
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passes through the wall of the quartz tube decreases as
time passes so that the W -curable resin on the surface
of the optical fiber is not homogeneously cured along its
length and the proper~ies of the resulting optical fiber
are adversely affected.
One object of the present invention is to provide a
method for producing an optical fiber having a coa~ing
layer made of an homogeneously cured W -curable resin
along its length.
Another object of the present invention is to mini-
mize the influence of the W-curable resin adhered and
carbonized on an inner surface of the quartz tube o~ a
W irradiating furnace.
According to the invention there is provided a
method ~or producing an optical fiber which comprises
the steps of: spinning an optical fiber from a molten
preform for an optical fiber, applying a UV curable
resin to the surface of the spun optical fiber to form a
resin coating, passing the fiber provided with the
2~ coating through a container defined by UV transparent
walls having inner and outer surfaces, containing a gas
for promoting the curing of the resin upon exposure to
W light, providing a layer of a fluid material outside
said walls of said container moving past said container
at a predetermined flow rate, said fluid material
containing at least two UV light-absorbing materials
which have different absorbing wavelengths, irradiating
the resin coating on said spun optical fiber with UV
light through said layer of fluid material and through
said walls of said container in order to cure said
resin, and adjusting the ~uantity of W light absorbed by
manipulation of a ratio of said at least two W light
absorbing materials in said fluid materials over time to
compensate for build up of W light-blocking material on
the insides of said walls, so that the quantity of W light
irradiating said W curable resin remains substantially
constant over time.
- 3a -
In the present invention, the term "a fluid" is
intended to include ~oth a liquid and a gas, as
appropriate.
The basis of the present invention is to cause the
W light to pass through a layer of W -absorbing material
so that the amount of light reaching the resin is reduced.
As the quartz tube becomes obstructed by the carbonized
resin, the amount of W -absorbing material is decreased,
so the total amount of the light reaching the resin re-
mains constant.
Referring to Figs. 3 and 4, in which the same numerals
as in Figs. 1 and 2 stand for the same elements, a W
irradiating furnace 5a further comprises a quartz tube 8a,
which has a larger diameter than the quartz tube 8 and is
concentrically installed around the quartz tube 80 In the
gap between the quart.z tubes 8 and 8a, a mixture of nitro-
gen and chlorine is supplied through an inlet pipe 8b and
exhausted through an outlet pipe 8c, whereby a layer of
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3~
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material for absorbing W light is formed. The gasses
are supplied from a system 12 (Fig. 5) for controlling
the flow rates of the gas~es.
The system 12 comprises a nitrogen gas cylinder 13,
a chlorine gas cylinder 1~, and a controlling circuit
15. The circuit 15 controls the flow rates of the
nitrogen and chlorine gas~es so that the ratio of the
gasses changes according to the line e shown in Fig. 9.
In place of the nitrogen and chlorine gasses, the
gaseous materials shown in Table 1 can be used as the
material for absorbing the W light. Table 1 also shows
the absorption peak wavelengths of W light for those
materials.
Table 1
Material Absorption peak wavelength (nm)
_ , ....... .. _
C12 330
, _ ,,
~2 290
. . . .. __ ___ ____.
. . ... _.____.___ _.__
2 A ._ . _ _____ _. _ _ ____. _ __ .__ __ ._ __ . _ . ___ __ . _._
3 256
H2O ~ ï67
The material for absorbing UV light may be used alone
or as a mixture with other material(s) in a suitable ratio.
The flow rates of the materials and optionally the ratio
of the materials are suitably changed during the resin
curing procedure so that the amount of UV light reaching
the resin remains constant and the resin is homogeneously
cured during the production of the optical fiber. The
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material used for absorbing the W light may be a liquid
or a gas.
In the preferred embodiment, a gaseous mixture of
nitrogen and chlorine is supplied to the space between
the quartz tubes 8 and 8a at a flow rate of 10 l/min.
while decreasing the content of the chlorine gas in the
mixture as time passes. The W light which reaches the
optical fiber during this period must pass through the
quartz tube 8, the gaseous mixture layer between the
quartz tubes 8 and 8a and the quartz tube 8a, and -the
amount of W light (330 nm) reaching the resin is as
shown in Fig. 6. The molar fraction o~ chlorine (x)
in the mixture of nitrogen and chlorine is expressed by
C12 (mole)
x =
N2(mole) ~ Cl2 (mole)
The relationship between the transmission o~ W light
and the molar fraction of chlorine in the mixture of nitro-
gen and chlorine is shown in Fig. 8, and the change of the
molar fraction of chlorine with time is shown in Fig. 9.
When the optical fiber coated with the W-curable
resin is irradiated by W light through only the quartz
tube 8 according to the conventional method, the trans-
mission of W light decreases as time passed as shown in
Fig. 7,
In the above embodiment~ the mixture of nitrogen and
chlorine is used as the material for absorbing W light.
However, when a material or a mixture of materials for
absorbing W light other than a mixture of nitrogen and
chlorine is used, th~ changing flow rate can
be easily determined by those skilled in the art by a
simple routine experiment.
As can be understood from the above, since the l~
light reaching the optical fiber passes not only through
the quartz tube 8 but also through the material for absorb-
ing the W light, the W light is uniformly irradiated on
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the optical fiber as time passes so that the W curable
resin is homogeneously cured along the length of the
optical fiber.
